Aerodynamics

Aerodynamic Data Analysis

The new Williams carbon clincher 45, 60 and 90mm rim designs were created using computational fluid dynamics(CFD). Our CFD driven rim designs were proven in the wind tunnel (data below) The result was the fastest wheels we have ever created.

Reading
the chart: The goal of any wheel design is to achieve the lowest drag
force as possible at all wind angles. The less drag, the faster the
wheel. Below is a chart highlighting drag force data.

Note: Drag force on the 45,60 and 90 wheels are very similar at a low wind (yaw) angle. As the wind angle increases, deeper section rims (90's) create less wind drag and slice through the air easier than a shallow section rim (45's).

Summary: Athletes looking for wheels that slice through air with the least amount of drag gravitate to deeper section wheels such as our System 60 and System 90 wheels. Athletes looking for a combination of lightweight and low wind drag gravitate to our System 45 and System 60 wheels.

Aerodynamics refers to the interaction between airflow and a moving object. At
Williams Cycling, our goal is to develop wheels that are designed to minimize drag and create wheel stability regardless of riding conditions. We use tools such as CFD and real world wind tunnel testing to
achieve our design goals.

Yaw AngleThe angle at which airflow (wind) interacts with a
wheel.For instance, a direct headwind
is considered a 0 degree yaw angle.Wind
blowing at a 20 degree angle would be considered a crosswind.

Types of WindMeteorological wind = wind that blows due to
weather.

·Resistance wind (thrust) = induced by the
cyclist by moving through air.For
instance, assume 0 meteorological wind and a cyclist riding at 20 mph, the
cyclist creates their own 20 mph headwind by moving through a still body of
air.

·Effective wind = the combination of
meteorological and resistance air.These
two winds combine to create one wind on the rider.

Resistance wind
dominates meteorological wind.On
average,athletes ride significantly
faster than the meteorological wind is blowing.Research modeling suggests that approximately 66% of wind yaw angles
experienced by riders are lower than 10 degrees. 30% of wind yaw angles are
between 10 to 20 degrees.The vast
majority of your riding will take place between 0 to 20 degrees yaw angle.

DragThe relative opposing force
imparted on an object as it moves against still air.It is the force exerted by airflow that
resists the forward motion of an object.Example, the faster you ride, the more opposing force you feel while
moving through air. The below pictures shows how air flows around a wheel at different yaw angles.

Laminar FlowLaminar flow is the air that moves across a wheel with no disruption or turbulence.

Stalled Air FlowAir that separates from the wheel and leaves pockets of
spiraling air in its wake. This leads to air flowing in
reverse direction which creates high drag and impedes movement.

Leading EdgeThe leading edge of a wheel
is the first object that meets airflow.Assuming
a direct headwind, or 0 degree yaw angle, the tire would be the leading
edge.Airflow hits the tire and evenly
flows around the rim.The rear fairing
at the back of the wheel is hidden from the wind.When wind yaw angle increases above 0 degree,
the back fairing is exposed to airflow and is considered the second leading
edge.

Side ForceWind
flowing past the surface of a rim exerts a force on it (total force). Lift (X force) is the component of
this force that is perpendicular to the oncoming wind flow direction.It
contrasts with the drag force (Y force), which is the component of the surface
force parallel to the flow direction. If the fluid is air, the force is called
an aerodynamic force.In other words, X force opposes Y force at a
perpendicular angle.In theory, when wind
yaw angle increases, lift force becomes greater than drag force.The more wind angle, the more lift force
occurs until critical angle of attack is reached (maximum lift). When wind
angle goes past critical angle of attack, the wheel stalls and no longer
benefits from side force lift.This is
when you feel the bicycle push you around on the road.Typically, this happens when there is
swirling wind and a wind gust blows into the cyclist at a sharp angle
(sideways).This can be a very
uncomfortable feeling for the cyclist.

Rim Shape (Airfoil)A deep section rim profile increases rim surface area.The larger the rim surface area, the more
wind side force.The goal of any rim
designer is to create an airfoil rim that maximizes side force and minimizes
drag.

Wheel Stability and Aerodynamic Drag
Aerodynamic performance is only one part of rim design.Wheel stability is critical for steering
control.To create a deep section
aerodynamic wheel that is stable in cross winds, we must equalize wind force on
the front (wind side) of the rim with the back (non-wind side) of the rim.

Torus vs. V Shape Rim

A torus is a donut shape rim.A V-Shaped rim is a V-shaped tori.

The
torus shaped rim is a low drag shape with regard to a bicycle wheel.This is because the leading edge of a rim
must also function as the trailing edge and vice versa.Low drag in both directions parallel to
motion equals good crosswind performance.

Ellipse (Torus) shape rims work very well in cross winds because they create
similar low drag in both directions parallel to motion. V-shaped rims created
low drag in one direction and high drag in the other direction parallel to
motion.